The invention relates to a hydraulic control assembly, which is used in particular for controlling hydraulic consumers in mobile machines.
One such hydraulic control assembly is known for instance from European Patent Disclosure EP 0 566 449 A1. This is a hydraulic control assembly on the load-sensing principle, in which an adjusting pump is set, as a function of the highest load pressure of the hydraulic consumers actuated, such that the pump pressure is above the highest load pressure by a defined pressure difference. The pressure medium flows to the hydraulic consumers via adjustable metering apertures, which are located between an inflow line leading away from the adjusting pump and the hydraulic consumers and are typically integrated with a main control valve serving to control the direction of a hydraulic consumer. By means of the pressure balances downstream of the metering apertures, it is attained that at an adequate quantity of pressure medium furnished by the adjusting pump, a defined pressure difference across the metering apertures exists, regardless of the load pressures of the hydraulic consumers, so that the quantity of pressure medium flowing to a hydraulic consumer is now dependent only on the opening cross section of the respective metering aperture. If a mobile machine is opened wider, then a higher quantity of pressure medium must flow across it in order to generate the defined pressure difference. The adjusting pump is adjusted in each case such that it furnishes the required quantity of pressure medium. This is accordingly also called demand flow regulation. For that purpose, the adjusting pump has a pump regulator, which can be subjected via a load reporting line to the highest load pressure of the simultaneously triggered hydraulic consumers. For limiting the pump pressure, a fixedly set pressure limiting valve is connected to the end portion, connected to the pump regulator, of the load reporting line, and this pressure limiting valve, in cooperation with a throttle restriction that decouples the end portion from the remainder of the load reporting line, limits the pressure reported to the pump regulator and thus also limits the pump pressure.
The pressure balances downstream of the metering apertures are urged in the opening direction by the pressure downstream of the respective metering aperture and in the closing direction by a control pressure prevailing in a rear control chamber; this pressure typically corresponds to the highest load pressure of all the hydraulic consumers supplied by the same hydraulic pump. If, when a plurality of hydraulic consumers are actuated simultaneously, the metering apertures are opened so widely that the quantity of pressure medium furnished by the hydraulic pump, which has been displaced as far as the stop is less than the total quantity of pressure medium supplied], then the quantities of pressure medium flowing to the individual hydraulic consumers are reduced in proportion, regardless of the load pressure at the various hydraulic consumers. This is accordingly called control with load-independent flow distribution (LIFD control). Since in LIFD control the highest load pressure is also sensed, and as a result of the variation in the quantity of pressure medium pumped, the hydraulic pump generates an inflow pressure that is above the highest load pressure by a defined pressure difference, LIFD control is a special case of load-sensing control (LS control).
When there is a plurality of hydraulic consumers, to which pressure medium flows via a respective metering aperture with an upstream pressure balance that is urged in the closing direction only by the pressure upstream of the metering aperture and is urged in the opening direction, via an individual load reporting line, only by the load pressure of the respective hydraulic consumer and by a compression spring, load-independent flow distribution is not obtained. In that case, only LS control and LS consumers are involved. Such control is known for instance from German Patent DE 37 09 504 C2. When a plurality of hydraulic consumers are actuated simultaneously and there is an inadequate quantity of pressure medium furnished by the adjusting pump, initially only the quantity of pressure medium flowing to the hydraulic consumer with the highest load pressure is reduced. When it stops, the quantity of pressure medium flowing to the consumer having the second-highest load pressure then decreases, and so forth.
In the hydraulic control assembly of German Patent DE 37 09 504 C2, an end portion, leading to the pressure balance is connected via a throttle restriction to the remainder of the individual load reporting line of a hydraulic consumer and to a pressure limiting valve. The latter is adjustable as a function of the magnitude of a pilot control signal serving to trigger the main control valve associated with the hydraulic consumer. The pressure balance now acts like a pilot-controlled pressure reduction valve, whose setting is variable by the pilot control signal, and which closes when a defined pressure is reached at its outlet. The pressure at which the pressure balance closes, and which prevails at a hydraulic consumer whose pressure balance is triggered accordingly on the closing side, can thus be limited individually for the consumer and varied via the pilot control signal.
In German Patent Disclosure DE 198 31 595 A1, an LIFD control is shown in which once again the pressure is limited individually for a hydraulic consumer. This requires that the rear control chamber of an LIFD pressure balance be constructively disconnected from the load reporting line. Also, a multi-way valve is necessary, as a function of whose switching position the rear control chamber communicates with the load reporting line or is subjected to pump pressure. The multi-way valve is switched as a function of the load pressure. No provision is made for varying the switching pressure during operation.